Add `matter_full_text_uri` property to `Matter` definition and impl

[evamaxfield]

While looking through the frontend and design work on the legislation tracking project, I realized for the first time that I think we may be missing a crucial piece of information which is a link to the actual full matter text.

Current Matter is defined as: link but a matter definitely has full text and we should store a link to that full text. I propose full_text_uri or some varient of that.

Additionally, while we look into this, it would be good to investigate which MatterFile's make it through the pipeline: https://github.com/CouncilDataProject/cdp-backend/blob/main/cdp_backend/pipeline/event_gather_pipeline.py#L1444

I think the above try-except block may be dropping some MatterFile / MinutesItemFile attachments that would be useful to keep and so we may want to try to fix it if we do see that behavior.